Re: [PATCH 2/6] rcu: Remove superfluous full memory barrier upon first EQS snapshot

Andrea Parri
2024-05-17T16:27:18Z

> Z6.0+pooncelock+poonceLock+pombonce.litmus shows an example of
> how full ordering is subtely incomplete without smp_mb__after_spinlock().
>
> But still, smp_mb__after_unlock_lock() is supposed to be weaker than
> smp_mb__after_spinlock() and yet I'm failing to produce a litmus test
> that is successfull with the latter and fails with the former.

smp_mb__after_unlock_lock() is a nop without a matching unlock-lock;
smp_mb__after_spinlock() not quite...

C after_spinlock__vs__after_unlock_lock

{ }

P0(int *x, int *y, spinlock_t *s)
{
int r0;

WRITE_ONCE(*x, 1);
spin_lock(s);
smp_mb__after_spinlock();
r0 = READ_ONCE(*y);
spin_unlock(s);
}

P1(int *x, int *y)
{
int r1;

WRITE_ONCE(*y, 1);
smp_mb();
r1 = READ_ONCE(*x);
}

exists (0:r0=0 /\ 1:r1=0)

> For example, and assuming smp_mb__after_unlock_lock() is expected to be
> chained across locking, here is a litmus test inspired by
> Z6.0+pooncelock+poonceLock+pombonce.litmus that never observes the condition
> even though I would expect it should, as opposed to using
> smp_mb__after_spinlock():
>
> C smp_mb__after_unlock_lock
>
> {}
>
> P0(int *w, int *x, spinlock_t *mylock)
> {
> spin_lock(mylock);
> WRITE_ONCE(*w, 1);
> WRITE_ONCE(*x, 1);
> spin_unlock(mylock);
> }
>
> P1(int *x, int *y, spinlock_t *mylock)
> {
> int r0;
>
> spin_lock(mylock);
> smp_mb__after_unlock_lock();
> r0 = READ_ONCE(*x);
> WRITE_ONCE(*y, 1);
> spin_unlock(mylock);
> }
>
> P2(int *y, int *z, spinlock_t *mylock)
> {
> int r0;
>
> spin_lock(mylock);
> r0 = READ_ONCE(*y);
> WRITE_ONCE(*z, 1);
> spin_unlock(mylock);
> }
>
> P3(int *w, int *z)
> {
> int r1;
>
> WRITE_ONCE(*z, 2);
> smp_mb();
> r1 = READ_ONCE(*w);
> }
>
> exists (1:r0=1 /\ 2:r0=1 /\ z=2 /\ 3:r1=0)

Here's an informal argument to explain this outcome. It is not the only
according to the LKMM, but the first that came to my mind. And this is
longer than I wished. TL; DR: Full barriers are strong, really strong.

Remark full memory barriers share the following "strong-fence property":

A ->full-barrier B

implies

(SFP) A propagates (aka, is visible) to _every CPU before B executes

(cf. tools/memory-model/Documentation/explanation.txt for details about
the concepts of "propagation" and "execution").

For example, in the snippet above,

P0:WRITE_ONCE(*w, 1) ->full-barrier P1:spin_unlock(mylock)

since

P0:spin_unlock(mylock) ->reads-from P1:spin_lock(mylock) ->program-order P1:smp_mb__after_unlock_lock()

acts as a full memory barrier. (1:r0=1 and 2:r0=1 together determine
the so called critical-sections' order (CSO).)

By contradiction,

1) P0:WRITE_ONCE(*w, 1) propagates to P3 before P1:spin_unlock(mylock) executes (SFP)

2) P1:spin_unlock(mylock) executes before P2:spin_lock(mylock) executes (CSO)

3) P2:spin_lock(mylock) executes before P2:WRITE_ONCE(*z, 1) executes (P2:spin_lock() is an ACQUIRE op)

4) P2:WRITE_ONCE(*z, 1) executes before P2:WRITE_ONCE(*z, 1) propagates P3 (intuitively, a store is visible to the local CPU before being visible to a remote CPU)

5) P2:WRITE_ONCE(*z, 1) propagates to P3 before P3:WRITE_ONCE(*z, 2) executes (z=2)

6) P3:WRITE_ONCE(*z, 2) executes before P3:WRITE_ONCE(*z, 2) propagates to P0 (a store is visible to the local CPU before being visible to a remote CPU)

7) P3:WRITE_ONCE(*z, 2) propagates to P0 before P3:READ_ONCE(*w) executes (SFP)

8) P3:READ_ONCE(*w) executes before P0:WRITE_ONCE(*w, 1) propagates to P3 (3:r1=0)

Put together, (1-8) gives:

P0:WRITE_ONCE(*w, 1) propagates to P3 before P0:WRITE_ONCE(*w, 1) propagates to P3

an absurd.

Andrea